Storage rings are commonly used to create synchrotron radiation for use in experiments in a variety of fields including chemistry and physics. This radiation is photonic in nature and spans the energy range from deep infrared to the hard x-ray. In order for the storage ring to function properly, it must be operated in the ultrahigh vacuum (UHV) pressure regime below 10.sup.-9 Torr. Many experiments, on gases or high vapor pressure materials, for example, are incompatible with UHV conditions, however, and must therefore be isolated from the storage ring and its ancillary beamlines. Thin windows are often used for this purpose, but photons in some portions of the synchrotron spectrum cannot effectively penetrate even the thinnest windows that are technologically possible.
In these cases, differential pumps have been used to provide the necessary isolation. FIG. 1A shows a schematic diagram to illustrate the principle of differential pumping. The pump diagrammed in FIG. 1A is a two-stage pump. The first stage consists of a first vacuum vessel 10 connected by a tube 12 of constricted dimensions to a second vacuum vessel 13 to which a pump 15 is attached. The second stage, which is a repetition of the first stage, is formed by a second constricted tube 17 connecting second vacuum vessel 13 to a third vacuum vessel 18, which also has an attached vacuum pump 20. As shown in the drawing, the pressures in the vacuum vessels are P.sub.1, P.sub.2, and P.sub.3, respectively. The pumping speeds of the two pumps are F.sub.1 and F.sub.2, and the conductances of the two tubes C.sub.1 and C.sub.2. In the situation as drawn, under the simplifying assumptions that the first vacuum vessel is the only source of gas present, that the pumping speeds of the pumps are much greater than the conductances of the constricted tubes, and that the system is operating in the molecular flow regime, then it is easy to show from standard vacuum formulae that the pressure in the third vacuum vessel is: EQU P.sub.3 =(C.sub.1 /F.sub.1)(C.sub.2 /F.sub.2)P.sub.1 ( 1)
EQN. 1 shows that, by using tubes with small conductances and pumps with large pumping speeds, very large pressure ratios may be obtained between the first and third vacuum vessels. For example, conductances of less than 1 liter/sec are readily obtained, as are pumping speeds in excess of 100 liter/sec. This would produce a pressure ratio of over 10.sup.4, allowing a vessel at 10.sup.-5 Torr to be connected to a vessel of 10.sup.-9 Torr.
FIG. 1B shows a cross-sectional view of an apparatus for implementing FIG. 1A, constructed using conventional UHV flanged stainless steel vacuum components. In this implementation the constricted tubes 12 and 17 of FIG. 1A are constructed of rectangular copper waveguide tubes 24 brazed into OFHC copper disks 25, which, in turn, are bolted to the flanged faces of two vacuum nipples 26. These assemblages are connected by vacuum tees 27 to schematically drawn ion pumps 28 as shown.
In the molecular flow regime vapor species (atoms or molecules) essentially never collide with one another. They travel in straight lines until they collide with some object, such as the vacuum vessel wall. After each collision they are reflected diffusely, departing from the point of impact in an essentially random direction relative to their direction of travel prior to the collision. The path followed by any particular vapor specie is thus a random walk, typically taking many steps to diffuse from one part of a vacuum system to another. FIG. 1B shows an example 29 of the complex path a molecule exiting a tube 24 might have to follow to reach a pump 28 and be removed from the system. This is why constricting tubes, such as 24, are effective in preventing molecular flow from one region to another: many reflections are required and in each reflection the molecule is as likely to make backward as forward progress. It also explains why, in each differential pumping stage, the closer the pump 28 is to constricting tube 24 the more effective the stage is, since the vapor species exiting the tube are more likely to reflect randomly into the pump before they reach the entrance to the constricting tube of the next differential section.
In the rest of this discussion only two-stage differential pumps will be discussed, since they are most commonly used in synchrotron applications. It should be clearly understood, however, that the same principles apply whether a single stage or multiple pumping stages are employed.
The principle of differential pumping has been applied to the synchrotron radiation problem by making the tubes in the successive pumping stages collinear so that light may pass through them on the path indicated by the line 22 shown in FIG. 1A. Third vacuum vessel 8 may then be connected to the storage ring or its beamline, while first vessel 10 can be connected to the experimental chamber. This allows the synchrotron light to pass from the third vacuum vessel to the first vacuum vessel along line 22 without passing through any physical barriers, yet also allows the required pressure difference to be maintained between the first and third vacuum vessels. It is not critical what type of pumps are used to supply the pumping action. Ion pumps, cryopumps, and turbo-molecular pumps have all been employed to implement pumps 15 and 20.
The prior art described above has one significant problem. This is that, while the synchrotron light passes from vacuum vessel 18 to vacuum vessel 10 along line 22, any vapor species (atoms or molecules) in vacuum vessel 10 which are traveling along line 22 in the opposite direction will pass unimpeded into vacuum vessel 18, since there is no physical barrier to their motion. While this beam is generally an insignificant fraction of the vapor species in first vacuum vessel 10, it may be a significant fraction in third vacuum vessel 18 because the pressure in third vacuum vessel 18 is so much lower. Furthermore, since this beam of vapor species is headed directly opposite to the synchrotron light, it will also impinge on any optics used to direct or focus the synchrotron light, where its constituent atoms or molecules may deposit or cause reactive damage. It would therefore be advantageous to have a differential pump which not only connected two vacuum regions along a line of sight, so that photons could freely pass between them, but also prevented vapor species from doing the same. The present invention addresses this problem by creating a mechanism which is capable of pumping any vapor species attempting to travel along the line 22, yet which is also completely transparent to the passage of photons of energies greater than the infrared.
In order to comprehend the operation of the invention pump it is necessary to understand the operation of prior art triode ion pumps which operate based on the Penning discharge mechanism. Two sectional views of the structure of such a triode ion pump are shown in FIGS. 2A and 2B. The pump comprises an electrode structure consisting of an anode 30 and a pair of cathodes 32 supported within a vacuum housing 37 which lies between a pair of magnets 38. The magnets fill the electrode volume with a magnetic field (B) 40 as indicated. The anode 30 comprises a 2-dimensional raft of short metal tubes which are usually spot welded together to form a rigid array. The tubes are typically about 2 cm in diameter by 3 cm long and made of stainless steel. The cathodes 32 are grids composed of long strips of a reactive metal such as Titanium or Tantalum, the strips each being typically about 0.5 mm by 4 mm in cross section. The figures do not show the structures which support the electrodes and isolate them electrically from each other and the vacuum housing, since these details are well known to those skilled in the art.
A simplified description of the operation the Penning mechanism is as follows, and may be understood by reference to FIG. 2C, which is a schematic diagram of a cross section through the electrode structure in a direction parallel to the cathode strips, as indicated in FIG. 2B. The cathode grids 32 are negatively biased compared to both the anode 30 and the vacuum housing 37, which are both usually at ground potential. Any electron finding itself anywhere between the two cathodes 32 is therefore attracted toward the wall of the nearest tube in the anode. As it attempts to move toward the tube wall, however, its velocity perpendicular to the magnetic field B produces an electromagnetic force at right angles to both its velocity and the field B, according to the well known "right hand rule." The result is that electrons are unable to reach the tube walls and instead form an electron plasma sheath surrounding them. Within this sheath each electron follows a looping orbit which, in the direction perpendicular to B, precesses about the inside of a single tube. In the direction parallel to B, where no tangential force is generated, the electrons are free to travel until they begin to leave the anode tubes 30 and approach the cathode grids 32. At this point they are repelled by the negatively biased cathode and forced back toward the anode. The electrons cannot escape from the "trap" thus formed and instead can only oscillate up and down between the grids and close to the tube walls. The resultant electron plasma sheathing the anode tubes is therefore stable and persistent. The nominal shape and location of the electron plasma 50 with respect to the anode tube walls is shown by dashed lines in FIG. 2C.
The electron orbits within the plasma sheath 50 are quite long lived and are terminated primarily by collisions with vapor species. These collisions are, in fact, the "ion pumping" mechanism, since each collision typically ionizes the vapor specie involved, which has two consequences. First, another electron is generated to join the pool of electrons in the electron plasma sheath 50 which are available to ionize further vapor species. Secondly, as indicated in FIG. 2C, where an atom V 52 is struck by an electron in the electron plasma, the ionized specie 53 feels an electric field from and is attracted toward the nearest (negatively biased) cathode grid 32. Its motion in this direction is essentially parallel to the magnetic field B 40 and is thus unimpeded. Any velocity perpendicular to B 40 generates an electromagnetic force, just as in the case of the electrons described above. As a result, the ion follows a helical orbit 55 from its point of generation until it collides with the cathode grid 32. As noted above, the grid is typically made of a chemically active material, such as Titanium or Tantalum. When the ion strikes the grid, it sputters neutral atoms N 57 of this material onto the surrounding pump walls, where other gas molecules can chemisorb to it, creating a secondary pumping mechanism. The ion itself is either be buried within the grid material by the force of its impact or, neutralized, can be chemisorbed there. In either case it has been removed from the vapor, which was the desired intent.
At a deeper level, Penning discharges are much more complex and the details of the electron plasma sheaths 50 surrounding the anode tubes 30 are not fully understood, in spite of numerous studies. It is known that the plasma tends to form a relatively dense layer, close to the anode tube walls and to shield their potential fairly effectively. The cathode grid 52 potential can thus penetrate into the anode tubes for significant distances along their axes, the exact distances depending in detail upon the applied voltage, magnetic field strength, and anode tube dimensions. As a result of this penetration, the electric field inside the tubes also has a strong radial component across the plasma sheath. To the extent that the cathode potential does not fully penetrate the anode tubes, the electron plasma is less dense and does not lie as close to the tube walls. The plasma sheaths 50 in FIG. 2C show this effect, thickening somewhat in the middle of the anode tubes 30.
In prior art, the inventor introduced a modification of the standard triode electrode geometry in order to create differential pumps with increased efficiency. In this design a triode ion pump was employed as first stage pump 15 in FIG. 1, and, as shown in FIG. 3, the standard triode electrode structure of FIG. 2A was employed with a single exception. The difference was that one or more rows of shortened tubes 60 were included in the raft of anode tubes 30. Shortening these tubes created a path whereby synchrotron light could pass through the length of the ion pump unimpeded. This allowed the pump F.sub.1 to be moved up until the line of sight 22 literally passed through it. Vapor species diffusing from first vacuum vessel 10 toward third region vacuum vessel 18 through constricted tube 12 are now injected directly into the active pumping region of the ion pump, greatly improving the efficiency with which they are pumped, since far fewer reflective bounces are required to reach the pump's active pumping region. Effective pumping speeds were increased by factors of approximately 10 by this stratagem, with a 110 liter/sec sized pump producing pressure drops which would have required a 1000 liter/sec pump in the traditional arrangement shown in FIG. 1. This improvement, however, does not address the line of sight problem described above, since it does not introduce any pumping mechanism for vapor species traveling in the region between the cathode grid 32 and the top of the shortened anode tubes 60. In particular, because the electron plasma sheaths lie close to the anode tube walls, they do not enter into this region and interact with the vapor species in it.